According to conventional technique, a thermoplastic resin is melted continuously under pressure in the barrel of an extruder having one or more screws, terminating with an extrusion head having a narrow extrusion orifice which may be flat (for sheets) or circular (for tubes). In an intermediate section of the barrel there is continuously injected into the molten resin a volatile expansion agent, usually in liquid state, for example a "Freon" (R.T.M.) or pentane and the extruder is designed in such a way as to produce a possibly uniform dissolution of the agent in the molten resin. The resin may also contain advantageously suitable nucleating agents, such as, for example, talcum, citric acid and sodium bicarbonate in the form of very fine particles uniformly dispersed in the molten resin. On the way to the extrusion orifice the molten material is subjected to a high pressure, which is necessary to prevent volatilization of the expansion agent. On leaving the extrusion orifice, the material undergoes decompression to atmospheric pressure, as a result of which the expansion agent separates within the material in the form of bubbles, giving rise to the desired foam.
It is known that the quality of the foam thus obtained is heavily dependent on the extrusion temperature. If the extrusion temperature is too high, the foam collapses or, at least, its specific gravity (density) is undesirably high in relation to the value theoretically obtainable, and its mechanical strength is poor. In principle, the higher the percentage of the expansion agent in the molten material, the lower the extrusion temperature should be, since otherwise the viscosity of the resin just extruded is insufficient to resist the disruptive pressure of the gas which is liberated in the resin. Since, in order to obtain foams of low density (less than 0.1 g/cc), substantial percentages of expansion agent are necessary, the problem of lowering of the extrusion temperature assumes great importance.